iwlwifi: software w/a for h/w bug cause Rx bit get clear

This is a w/a for a hardware bug. the h/w bug may cause the Rx bit
(bit 15 before shifting it to 31) to clear when using interrupt coalescing.

This does not mean frames are lost - their processing is just delayed until
next interrupt arrives.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>